Project help for a n00b... :(

Hey guys, I got thrown into this class for electronics and it's final project time, I was simply trying to create an arm that goes up to the high input, and back down on the low input. That's the arduino part of my project but I definitely need a power supply to power the arm. I used this link that I found in the forums: http://arduino.cc/playground/Main/DCMotorControl does anyone have any other input on this that could help me out? That would be a great help :)

One obvious question is “how big is the arm?”